The present invention relates to grain-oriented magnetic steel sheets having extremely low iron loss which are suitable for use as iron core materials for transformers and power generators, and to methods for producing the same.
It is well known that grain-oriented magnetic steel sheets which contain Si and whose crystal orientations are (110) [001] and (100) [001] have excellent soft magnetic properties. Therefore, they are widely used as iron core materials for various types of electrical apparatuses used in the commercial frequency band. In general, in such a grain-oriented magnetic steel sheet, the iron loss (W17/50), which is observed when magnetized at 1.7 T at a frequency of 50 Hz, must be low.
Core loss consists of eddy current loss and hysteresis loss. Various methods are known in order to effectively decrease eddy current loss. Examples thereof include a method of increasing electrical resistance by adding Si, a method of decreasing steel sheet thickness, and a method of decreasing grain size. On the other hand, in order to effectively decrease hysteresis loss, a method of aligning crystal orientation is known.
However, the method of increasing electrical resistance by adding Si has limitations. This is because the saturation magnetic flux density decreases if an excessively large amount of Si is added. The decrease in the saturation magnetic flux density results in an increase in the size of iron cores, which is undesirable.
The method of decreasing steel sheet thickness also has limitations. This is because the rolling loads greatly increase, resulting in an extreme increase in manufacturing costs.
With respect to the method of aligning crystal orientation, products which have a magnetic flux density (B8) that is close to the saturation value, such as 1.96 T or 1.97 T, have already been obtained. Therefore, there is little room for a further decrease in hysteresis loss.
Recently, techniques for reducing iron loss by artificially decreasing magnetic domain widths have been developed. Examples thereof include a method of locally introducing strain by applying plasma jet or laser beams to the surfaces of steel sheets and a method in which grooves are formed in the surfaces of steel sheets. Although a considerable iron loss reduction effect has been achieved by such magnetic domain-refining techniques, the effect has also reached its limit.
As the other techniques for reducing iron loss, methods related to the surface properties of magnetic steel sheets have also been disclosed.
Japanese Examined Patent Application Publication No. 52-24499 discloses a method in which the roughness at the interface between a metal surface of a steel sheet and a surface of a non-metallic coating film is decreased. Japanese Examined Patent Application Publication Nos. 7-9041, 5-87597, and 6-37694 disclose methods of performing so-called xe2x80x9ccrystal orientation-intensifying treatmentxe2x80x9d in which crystals with specific crystal orientations are left on metal surfaces.
In order to reduce iron loss by such methods, strong tension must be applied to a steel sheet. Therefore, it was required to form a tensile coating film on the surface of the steel sheet. If no tensile coating film was provided, since the surface of the steel sheet was smooth, an increase in magnetic domain width was accelerated, resulting in a large increase in iron loss.
In order to overcome this problem, the Japanese Examined Patent Application Publication No. 52-24499 teaches a method in which the surface of the steel sheet is subjected to chemical polishing or electrolytic polishing to form a specular surface, and the surface of the steel sheet is further thinly metal-plated. The method aims to suppress an increase in iron loss by preventing the oxidation of the surface of the steel sheet and by preventing the deterioratation of the surface smoothness of the steel sheet during coating and baking of the insulating coating film.
However, when metal plating had tension, the insulating coating film was likely to be peeled off by baking treatment. Even if the insulating coating film avoided being peeled off, since the insulating coating film was an ordinary phosphate non-tensile insulating coating film, reduction in iron loss was small.
When metal plating did not have a tensile effect, the iron loss reduction was very small. Moreover, even if a phosphate non-tensile insulating coating film was intended to be formed, the adhesion of the coating film was not satisfactory, and it was not possible to reduce iron loss.
Japanese Unexamined Patent Application Publication No. 62-103374 discloses a method in which a mixed ultrathin layer of the matrix and a variety of oxides, borides, silicides, phosphides, or sulfides is formed on a steel sheet surface smoothed by polishing, and a baked insulating coating film is further formed thereon. This method provides excellent adhesion between the steel sheet and the insulation film. However, since the mixed ultrathin layer of the matrix is present, the iron loss reduction effect due to mirror finishing of the steel sheet surface disappears, resulting in a small reduction in iron loss.
Japanese Unexamined Patent Application Publication No. 2-243770 discloses a method in which a ceramic coating film is formed by a sol-gel process. However, in this method, since adhesion with the steel sheet is inferior, it is not possible to provide a satisfactory tension-applying effect to the steel sheet, resulting in a small reduction in iron loss.
Japanese Examined Patent Application Publication No. 56-4150 discloses a method in which a steel sheet surface is smoothed by chemical polishing or electrolytic polishing so as to have a centerline mean roughness (Ra) of 0.4 xcexcm or less, and a ceramic thin film is further formed thereon. However, since the ceramic thin film having satisfactory adhesion is formed by vacuum deposition, a large apparatus are required. The deposition rate is also low, and this method is not suitable for industrial production.
Japanese Unexamined Patent Application Publication Nos. 3-47957, 3-294465, 3-294466, 3-294467, 3-294468, 3-294469, and 3-294470 disclose methods in which coating films of oxides or silicides are formed by low pressure plasma spraying on the surfaces of smoothed matrices or the surfaces of metal-plated films formed thereon. Japanese Unexamined Patent Application Publication No. 10-245667 discloses a method in which tensile coating films composed of oxides, nitrides, or carbides are formed by plasma spraying. In these methods, although industrially acceptable deposition rates are ensured, since film formation is performed by droplet deposition, it is not possible to form dense films. Moreover, the coating films have rough surfaces, and peeling occurs easily by friction. Furthermore, adhesion between the surfaces of the steel sheets or plated surfaces and the oxide or silicide coating films is not satisfactory. Therefore, a reduction in iron loss is insufficient. Similarly to vacuum deposition, large-scale pressure reducing apparatuses are required.
As described above, in recent techniques for reducing iron loss of grain-oriented magnetic steel sheets, it is absolutely necessary to form a tensile coating film on the steel sheet surface after smoothing the steel sheet surface during finishing annealing or in the subsequent treatment and/or after crystal orientation-intensifying treatment is performed. However, since the tensile coating film applies strong tension to the steel sheet surface, strong shearing stress is applied to the interface between the steel sheet surface and the tensile coating film, and the coating film is easily peeled off. As a result, tension application cannot be achieved, resulting in an insufficient reduction in iron loss.
Accordingly, various ideas have been adopted in order to secure adhesion of tensile coating films. However, when the adhesion was satisfactory, the iron loss reduction effect by smoothing of the steel sheet surface was lost.
Furthermore, when crystal orientation-intensifying treatment is performed to the steel sheet surface, the adhesion of the tensile coating film is slightly eased compared to smoothing treatment. However, in such a case, the adhesion is still far from the desired level, and the steel sheet did not have a sufficient tensile effect, and the reduction in iron loss was not satisfactory.
That is, the recent techniques for reducing iron loss of grain-oriented magnetic steel sheets have not been industrially used yet.
The present invention advantageously solves the problems described above. That is, it is an object of the present invention to provide a method for producing a grain-oriented magnetic steel sheet in which the surface of the steel sheet is subjected to smoothing treatment or crystal orientation-intensifying treatment and tension is applied to the steel sheet by a tensile coating film so as to greatly reduce iron loss and in which tensile force is sufficiently acted on the steel sheet without impairing the adhesion of the tensile coating film. It is another object of the present invention to provide a grain-oriented magnetic steel sheet having extremely low iron loss obtained by the method.
The development of the present invention will be described below.
The present inventors have carefully investigated the cause of the peel-off of the materials formed by conventional low pressure plasma spraying or the materials formed by ordinary plasma spraying. The following results have been obtained.
a) The coating material formed, i.e., coating film, is not a dense, single layer and has a structure in which particulate substances stick to each other in sequence, resulting in voids between particles. Such a structure is formed due to droplet spraying.
b) The coating film is easily peeled off because of such a structure. This is because cracking may start to occur from the voids in the coating film and the oxidation of the surface of the steel sheet through the voids may cause the peel-off of the coating film. The irregularities of the surface also accelerate the peel-off of the coating material.
The voids are produced between individual particles because droplets in a semi-molten state are jetted onto the surface of the steel sheet, and flat particles which stick together in sequence are overlaid to form the coating film. So-called xe2x80x9ccold jointsxe2x80x9d are formed between particles, resulting in a decrease in strength. Irregularities resulting from the individual particles remain in the outermost surface. Furthermore, the oxidation of the surface of the steel sheet through the voids between particles accelerates the peel-off of particles.
On the assumption that deposition of a coating material in a vapor phase might solve the problems described above, the present inventors investigated various coating film formation methods. In a vacuum deposition process and a vapor phase synthetic process, which are well-known vapor-phase processes, the reduction in iron loss was not satisfactory. On the other hand, in a thermal plasma vapor deposition process, in which a coating material is deposited in a plasma state, that is a kind of the vapor phase, iron loss was satisfactorily reduced. While a coating layer is deposited at a very low deposition rate in an atmosphere close to a vacuum in the former case, a coating layer is deposited at a high deposition rate in an atmospheric atmosphere or in a slightly reduced pressure atmosphere in the latter case. Under the latter conditions, it was considered that since a dense coating layer having a small porosity was formed with an appropriate thickness, adhesion was satisfactory, and thus a satisfactory reduction in iron loss was achieved. Moreover, in the latter process, since a large-scale vacuum chamber is not required and the deposition rate is high, industrial advantages are expected.
The present inventors have carried out investigations further and found more detailed conditions. That is, it has been found that by forming a coating layer with a mean thickness of not less than 0.01 xcexcm and not more than 10 xcexcm on a surface of the matrix of a steel sheet with a thickness of 0.27 mm or less by vapor deposition in a low oxidizing atmosphere with an oxygen partial pressure of less than 0.1 atm and a total pressure of 0.1 atm or more, a grain-oriented magnetic steel sheet having extremely low iron loss is produced. It has also been found that the coating layer formed is dense with a porosity of 10% or less and also has a smooth surface, satisfactory adhesion of the coating film is obtained, and a large reduction in iron loss is achieved. It has also been found that by performing magnetic domain-refining treatment before or after vapor deposition, a further reduction in iron loss is enabled, which is preferable.
With respect to small iron cores, since deterioration in iron loss due to shear strain after shearing process is a problem, in many cases, customers perform stress relief annealing. The stress relief annealing is usually performed at approximately 800xc2x0 C. for approximately 3 hours. In some cases, the coated layers were not able to resist the stress relief annealing, and the coating layers were peeled off. As a result, tension was not applied to the steel sheets, and rather iron loss was made worse greatly. Further investigations have been carried out on this problem, and it has been newly found that by forming a coating film on one surface of the steel sheet by controlling the deposition rate in the range of not less than 0.02 nm/sec and not more than 50 nm/sec, it is possible to produce a grain-oriented magnetic steel sheet having excellent adhesion of the coating film and therefor extremely low iron loss even after stress relief annealing is performed.
It has also been found that by controlling the temperature of the magnetic steel sheet at 200xc2x0 C. or more during vapor deposition, or by performing heat treatment to the steel sheet at 200xc2x0 C. or more after vapor deposition, adhesion is further improved. It has been found that by using fine powder with an average particle size of 5 xcexcm or less as a depositing material, the irregularities on the surface of the steel sheet are reduced, and further improvement in adhesion is expected.
The present invention has been achieved based on the findings described above.
That is, in one aspect of the present invention, a method for producing a grain-oriented magnetic steel sheet having extremely low iron loss includes the step of forming a coating layer on a surface of the matrix of a grain-oriented magnetic steel sheet having a thickness of 0.27 mm or less by vapor deposition in a low oxidizing atmosphere with an oxygen partial pressure (Po2) of less than 0.1 atm and a total pressure of 0.1 atm or more. Preferably, the coating layer is composed of any one of a chalcogenide, a silicide, a boride, and a nitride, and the coating layer on one surface of the steel sheet has a mean thickness of not less than 0.01 xcexcm and not more than 10 xcexcM. Preferably, the vapor deposition is performed by a thermal plasma vapor deposition process, the temperature of the steel sheet is controlled at 200xc2x0 C. or more during vapor deposition or after deposition, or fine powder with an average particle size of 5 xcexcm or less is used as a depositing material. In order to extremely reduce the iron loss after stress relief annealing, preferably, the vapor deposition is performed at a deposition rate of not less than 0.02 nm/sec and not more than 50 nm/sec. Other iron loss-reducing treatment, such as smoothing treatment or crystal orientation-intensifying treatment to the surface of the steel sheet, or magnetic domain-refining treatment before or after vapor deposition may be further performed. In another aspect of the present invention, a grain-oriented magnetic steel sheet having extremely low iron loss with a thickness of 0.27 mm or less includes a coating layer formed by vapor deposition on a surface of the matrix of the steel sheet, the coating layer being composed of a material that has a smaller coefficient of thermal expansion than that of iron, the coating layer having a porosity of 10% or less, The coating layer having mean thickness for one surface of not less than 0.01 xcexcm and not more than 10 xcexcm, and the coating layer having a centerline mean roughness (Ra) of less than 0.5 xcexcm. The magnetic domain-refining treatment may be performed to the steel sheet.